Maybe, but the board had good ENIG with no visible signs of corrosion. Perhaps the conductive polymer pads degraded? I still like my evaporation/condensation theory on account of it involving the one major thing Keithley did differently from everyone else (adhesive). They had an industry standard proven setup and they changed one thing, so naturally I suspect it. Diffusion through the adhesive region would be slow on normal timescales, yes, but this happened over years, and having some goopy organic residue covering the contacts seems like it would create the observed symptoms. This is all just speculation, though. What I know is that wiping and scrubbing both sides with IPA fixed the problem.
@arcitech, no, I bought this DMM7510 a few years back. The seller didn't disclose the exact problem, but they marked it For Parts/Repair, so it could have been worse